1. Field of the Invention
The present invention relates to a color thermal printer and a color thermal printing method to convey a recording sheet and record a color image thereto, and more particularly to a color thermal printer and printing method to convey the recording sheet stably by decreasing tension applied to a conveyor roller pair.
2. Description Related to the Prior Art
A color thermosensitive printer, utilized as a color thermal printer, has a thermal head and a conveyor roller pair. While the conveyor roller pair conveys a color thermosensitive recording sheet (hereinafter referred to as recording sheet), the thermal head records a full-color image to the recording sheet. During the image recording, the thermal head presses the recording sheet and applies heat onto yellow, magenta and cyan thermosensitive coloring layers of the recording sheet.
There are various kings of the color thermosensitive printer, such as a three-pass type and a one-pass type. The three-pass type color thermosensitive printer has a single thermal head, and conveys the recording sheet forward and backward by three times to record a full color image. On the other hand, the one-pass type color thermosensitive printer has plural thermal heads to record a color image while the recording sheet is conveyed forward once, thereby it takes shorter time to record an image than the three-pass type printer. Thus, the one-pass type thermosensitive printer is suitable for business use that requires sequential image recording.
The one-pass type color thermosensitive printer, however, have to move thermal heads successively to press the recording sheet in accordance with the conveyance. Pressing the recording sheet increases tension to the recording sheet and the conveyor roller pair. Increase in tension changes the conveyance speed of the recording sheet, which causes unevenness in the recorded image. In order to prevent unevenness, the color printer described in EP 1 036 662 A2 (corresponding to JP-A 2000-263864, equivalent to U.S. Pat. No. 6,474,886) provides a set of tension rollers near thermal heads to apply tension. The tension roller changes tension to the recording sheet when the leading end of the recording sheet passes the thermal head in the downstream side, so as to prevent fluctuation in tension to the recording sheet.
FIG. 7 shows a relationship between the conveyance speed and tension to the conveyor roller pair. In order to convey the recording sheet stably, it is necessary to keep the tension with in a stable range shown in FIG. 7, which is found to be about ±1 kgf according to experiments. The drive power of the conveyor roller pair is so strong that forward tension to the conveyor roller pair easily exceeds the narrow stable range, even if grid rollers are used as the conveyor roller pair.
Moreover, the coefficient of friction is variable depending on image density, temperature, humidity and so forth. Fluctuation in the load to the conveyor roller pair because of forward tension and the coefficient of friction causes change in the conveyance speed of the recording sheet. In that case, the recording position of each line is shifted to cause unevenness in the recorded image.